The Phenomenon of Evidence: Article by David Boersema (Pacific University).
Intro: >I want to begin with two stories, as it were, though by “stories” I do not mean they are fictional. Indeed, they are not. First story: Stanford philosopher John Perry tells of a time he once followed a trail of sugar on a supermarket floor, pushing his cart down the aisle on one side of a tall counter and back the aisle on the other, seeking the shopper with the torn sack to tell him he was making a mess. With each trip around the counter, the trail became thicker, but Perry seemed unable to catch up with the offending shopper. Finally it dawned on him that he was the person he was trying to catch.
Second story: In the late 1970s, Berkeley geologist Walter Alvarez came across an unusual layer of clay near the town of Gubbio, Italy. It was a layer of clay only a few inches thick and lay stratigraphically at the boundary between two recognized geological periods, the earlier Cretaceous and the later Tertiary, a boundary known to geologists as the K-T boundary, formed approximately 65 million years ago. An analysis of this clay boundary revealed that it contained a small amount (9.4 parts per billion) of a rare inert metallic element, iridium. As small as this amount was, it was 300 times the levels of iridium that was expected, given the levels found on either side of the K-T boundary. As relatively high levels of iridium (up to 500 ppb) are known to exist in meteorites, an extra-terrestrial origin of the Gubbio iridium level was hypothesized and, along with other data, this information led in 1980 to the hypothesis that the mass extinction of life on earth that occurred 65 million years ago was the result of bolide impact (i.e., of a meteorite or other celestial body striking the earth). Many geologists, skeptical of this impact hypothesis, claimed that such a high iridium spike does not entail bolide impact and that, in fact, it could be accounted for by terrestrial volcanic activity. To support this claim, Dartmouth geologist Charles Officer noted that the 1983 eruption of Mount Kilauea in Hawaii resulted in an iridium deposit that was 11,500 times the background concentration. Indeed, many geologists have pointed to the K-T iridium spike (which is found at numerous sites around the world) and to iridium spikes or “extended deposits” at other stratigraphic points in the geologic column to bolster their contention that the K-T mass extinction and other mass extinctions were the result of terrestrial vulcanism not bolide impact.
What do these two stories have to do with each other and what do they have to do with issues relating to evidence?<
Link
posted by johannes,
Monday, February 07, 2005
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